1. Field of the Invention
The present invention relates to an autocatalytic electroless gold plating method that is capable of giving an adequate plating rate and continuous thick gold over a wide pH range from acidic to neutral conditions. In particular, it relates to an electroless plating solution that is suitable for industrial use and is capable of giving an adequate plating rate and a continuous thickness of gold by an autocatalytic electroless reaction even under acidic conditions, which are not often employed conventionally because the reducing power of a reducing agent is weakened.
2. Description of Related Art
In recent years, electronic equipment such as cellular phones has been developed so as to have a small size, multiple functions, and high performance. This is due to ‘high integration and microfabrication of semiconductor chips’, which is a core technology, and also to progress in ‘packaging technology’ for packaging the device by external connection and mounting on a substrate. Because of increases in fineness and complexity of wiring of printed wiring boards and semiconductor chips, an increase in isolated circuits, an increase in fineness and a decrease in pitch of internal and external connecting terminals, electroless gold plating has been employed instead of gold electroplating, which requires leads for carrying current. The electroless gold plating employed here is a plating method that enables the deposition of soft gold suitable for wire bonding or flip chip connection.
With regard to the autocatalytic electroless gold plating solution, in terms of the bath composition, two types have been developed and put to practical use, that is, a cyanide-based bath, and a cyanide-free bath, which employs no cyanide. The cyanide-based plating solution is currently used in a wide range of applications since it has the advantages of gold cyanide complex stability, low cost, etc. For example, a cyanide-based plating solution containing a gold cyanide salt (KAu(CN)2, etc.) as a gold salt and an alkali metal tetrahydroborate (KBH4, etc.) or DMAB: dimethylamine-borane ((CH3)2NHBH3) as a reducing agent has been developed by Okinaka (Plating, 57, 914 (1970)).
A gold cyanide complex used as the gold salt in the cyanide-based bath is the most stable complex (Au(CN)2− complex stability constant: 1039) among currently known gold complexes. In order to make gold deposit from this gold cyanide complex, it is necessary to use a strong reducing agent such as DMAB, and at the same time employ high temperature and highly alkaline operating conditions, but with regard to the physical properties of the gold thus deposited, it is high purity soft gold and suitable for wire bonding, etc. However, since the operating conditions involve high temperature, strong alkali, a large amount of highly toxic free cyanide, etc., there are the problems that they cannot be applied to a material that is susceptible to alkali, such as polyimide or aluminum nitride, and a semiconductor material equipped with a positive photoresist cannot be plated.
In order to solve such problems, some cyanide-based electroless gold plating solutions employing various reducing agents that can be operated under acidic or neutral conditions have been reported (Denkimekki Kenkyukai, Electroless Plating—Basics and Application, The Nikkan Kogyo Shimbun, Ltd., 1994, 167–168, and JP, A, 59-85855).
However, these plating solutions have the problems that they are difficult to handle under operating conditions of about 95° C., the solution lifetime is extremely short, etc. In particular, a plating solution employing ascorbic acid as the reducing agent has a slow plating deposition rate due to its low reducing power and cannot be put to practical use. Judging from the situation that no cyanide-based autocatalytic electroless gold plating bath for use under acidic to neutral conditions is currently commercially available, no autocatalytic bath that can be used in practice under acidic to neutral conditions has actually been developed.
On the other hand, as a method for increasing the plating rate, there is a known method in which ions such as Pb ions or Tl ions are contained in a plating solution (JP, A, 60-125379), but a cyanide-based autocatalytic electroless gold plating bath containing ascorbic acid as the reducing agent and Pb ions or Tl ions as a deposition accelerator in a plating solution is not currently known.